1. Field of the Invention
The present invention relates to an electronic-type endoscope system provided with a changeover means for effecting changeover of the gain variation region of an automatic gain control means in accordance with the kind of electronic endoscope employed.
2. Description of the Related Art
In recent years, electronic-type endoscope systems having imaging means constituted by a solid state imaging device such as a charge-coupled device (hereinafter referred to as the "CCD") have been widely used. Such electronic-type endoscope systems fall into two major types: the type which employs an electronic endoscope (or electronic scope) whose front end portion includes such an imaging means and the type which employs an optical endoscope whose eyepiece portion is externally provided with a television camera including such an imaging means.
In such an electronic-type endoscope system having the imaging means for effecting photoelectric conversion, it is possible to easily implement signal processing of the output signals of the imaging means or to record them on a VTR or image file apparatus.
The current trend in the field of electronic-scope applications is to appropriately apply electronic scopes of various types, from a small-diameter type to a large-diameter type, to different portions to be observed. Accordingly, it has been proposed to use various kinds of built-in solid state imaging devices in accordance with the outer diameters of the inserting sections of electronic scope employed.
Where an electronic-type endoscope system employing one signal processing system is combined with an electronic scope employing an imaging means consisting of a plurality of solid state imaging devices, it is necessary to establish the system's environment in accordance with the kind of solid state imaging device.
Japanese Patent Laid-open No. 211040/1987 discloses an example of a prior art arrangement in which the requisite function can be selected in accordance with the kind of endoscope connected.
In the above-described prior art arrangement, initialization as to the execution or non-execution of image reversal, the presence or absence of a mask configuration, and so on, is carried out in accordance with the kind of endoscope connected. However, it has been impossible to combine the prior art arrangement with imaging means of various types each employing a solid state imaging device consisting of a different number of pixels.
Different types of optical systems are used in electronic scopes which employ imaging means consisting of solid state imaging devices having different numbers of pixels. In general, since such an electronic scope has a fixed focus, it is necessary to increase the f-number of the optical system disposed in front of the light receiving surface of the solid state imaging means in order to obtain a substantially focused image at a distance sufficiently long to allow observation. As the resolution of an image is enhanced, the f-number must be increased. If the f-number is increased, the quantity of light obtained at the light receiving surface of the solid state imaging device decreases even if the intensity of illumination of an object is kept constant. As a result, a deficiency of the quantity of light may easily occur. For this reason, an automatic gain control (hereinafter referred to as the "AGC") circuit is provided, and the AGC circuit serves to increase the gain of an image signal and set the level of the image signal to a proper level.
If a decrease in the quantity of light obtained at the light receiving surface of the solid state is compensated for by the above AGC circuit, particularly if the f-number is increased (for the case of high resolution), severe noise tends to occur.
Since the signals of high-resolution images contain high-frequency components, severe high-band noise tends to appear.
If the gain variation region of the AGC circuit is fixed, the use of an electronic scope whose resolution is not too high will enable an image to be displayed at a low noise level. However, for the above-described reason, if a high-resolution electronic endoscope is employed, an image containing severe noise may be reproduced and consequently the result of endoscopic inspection may be adversely affected. In addition, since the noise level differs, an operator may be afraid that his endoscope is an imperfect product.